While preparing the site prior to construction, all roots, stumps, dead timber, and other wood debris should be re-moved from the site. Similarly, wood scraps from construc-tion should be properly disposed. Leaving this material on site or in the backfill provides additional food sources for termites, attracting them and increasing the likelihood of infestation. Similarly, wood grade stakes or bracing stakes should be removed before or during a concrete placement and not be cast into the concrete. Leaving them in place attracts termites and provides a direct path for them through the concrete. Refer to Figure 2 for a summary of critical termite access areas.
Backfilling with a free draining soil, incorporating a subgrade drainage system, and installing proper above-grade water drainage will help keep the foundation and adjacent soil dry, providing a less hospitable environment for termites.
In extreme circumstances, subterranean termites may not require constant access to and from the adjacent soil.
Where conditions exist such that wood remains continu-ously wet, termites do not need to return to the soil to obtain water. However, such conditions are rare if proper design and construction for water penetration resistance are adhered to.
REDUCING ENTRy ROUTES
Once the termites have established a path, they have unimpeded access to the entire structure. Therefore, keeping termites out of the structure should always be the paramount objective. In addition to the obvious points of entry, such as wood in direct contact with the soil, other obscure (but criti-cal) termite entry routes include:
• through cracks in exposed wall faces or slabs. Termites are capable of moving through a crack only 1/32 inch (0.79 mm) wide;
• direct access from soil under porches or patio slabs;
• along the outside of pipes penetrating slabs or foundation walls; and
• access tunnels on the interior or exterior of walls.
Minimum Clearance to Soil
It is desirable to keep wood elements as far as possible from the soil to minimize termite access. Nonstructural wood elements, such as wood siding and trim, should be kept a mini-mum of 6 inches (152 mm) from the soil surface. Structural wood framing, sill plates, and sheathing should be kept at least 8 inches (203 mm) above the soil, or as otherwise required by local building codes. However, if the nonstructural wood is in contact with the structural wood (which is generally the case), the 6 inch (152 mm) minimum clearance should be increased to 8 inches (203 mm). These general clearances do not apply
Key Notes
1 – Ensure that the soil directly adjacent to the foundation is dry and free of scrap lumber or decaying wood.
2 – All utility penetrations through foundation walls should be sealed for both termite and water penetration resistance.
3 – Remove any dead or decaying wood from the area. All trees and plants should be healthy.
4 – Any wood in direct contact with the ground should be rated for such use. Otherwise untreated or not naturally termite resistant wood provides a direct path for termite passage.
5 – Inspect the foundation at regular intervals for signs of termite activity or the development of cracks.
Figure 2—Concerns Regarding Termite Protection
87
to pressure-treated wood or other termite and decay resistant woods.
Minimizing Cracks
Proper structural design of foundation walls, footings, and slabs will help prevent structural cracking that may al-low termite entry. In addition to preventing cracks due to structural overload, cracking due to concrete shrinkage also needs to be addressed. Due to fluctuations in the temperature and moisture content, all materials have a tendency to expand and contract over time. With concrete masonry founda-tions, the primary concern focuses on shrinkage resulting in the development of tensile stresses. This is because the tensile strength of concrete is relatively small compared to the compressive strength; therefore shrinkage may result in small cracks within the masonry.
It is normally not necessary to provide control joints in below grade residential concrete masonry basement walls.
A control joint is a planned joint in a concrete masonry wall at regular intervals that accommodates shrinkage movement without unsightly, random cracking. The lack of a need for control joints is attributed to the relatively low range of ther-mal and moisture fluctuations occurring in below grade walls afforded by the soil adjacent to the walls and to the water resistant systems applied to basement walls. In most below grade basement wall construction, it is possible to provide a reinforced bond beam at or near the top of the wall in lieu of control joints to minimize crack development. The bond beam also provides a cap, preventing termites from coming up through the empty cores of ungrouted block and gaining entry into the building. Joint reinforcement embedded in the horizontal bed joints, usually at 16 inches on center, also provides additional tensile strength for the masonry and aids in crack control. It should be pointed out that horizontal reinforcement will not completely eliminate cracking, but it will hold the cracks so tightly together that the termites cannot get through.
Additional measures to reduce the shrinkage cracking potential of concrete masonry include keeping the walls dry during construction. Because drying shrinkage is a primary cause of cracking in concrete masonry walls, it is important to minimize the potential for wetting concrete masonry dur-ing the construction process. At the jobsite, concrete block should be stored so as to protect the units from absorbing ground water or precipitation. This includes storing block on pallets (or otherwise isolating block from direct contact with the ground) and covering the units with plastic or other water-repellent materials.
Concrete masonry units should be dry when laid. Some surface moisture is acceptable; however, saturated units should be allowed to dry out before placement in the wall.
Concrete masonry units should never be wetted before or during placement in the wall, as may be customary with other types of masonry units.
At the end of each workday, a weatherproof membrane should be placed over uncompleted walls to protect the units from rain or snow. Placing a board on top of the membrane
will help hold it in place and will prevent the membrane from sagging into the masonry cores and allowing water to collect. To limit concrete slab cracking, the recommenda-tions of the American Concrete Institute (ref. 5) for quality concrete placement should be followed.
In basement walls, the dampproofing and waterproofing measures employed to reduce water penetration aid in the prevention of termite entry. Waterproofing and dampproof-ing systems require that the barrier be continuous to prevent water penetration into voids or open seams. Similarly, the barrier is typically carried above the finished grade level to prevent water entry between the barrier and the foundation wall. Cracks exceeding 0.02 inches (0.5 mm) should be repaired before applying a waterproof or damp-proof bar-rier. However, the repair of hairline cracks is typically not required, as most barriers will either fill or span these small openings. In addition, waterproofing and dampproofing systems should be applied to clean dry walls. In all cases, manufacturer’s directions should be carefully followed for proper installation.
Particular attention should be paid to reentrant corners at garages, porches, and fireplaces and to wall penetrations.
Because stress concentrations develop at these intersections, pliable membranes and/or additional reinforcement are often recommended at these locations to span any potential cracks or hold them tightly together.
Typical water penetration measures include coatings, sheet membranes, and drainage boards. Coatings are sprayed, trowelled, or brushed onto below-grade walls, providing a continuous barrier to water entry. Coatings should be applied to clean, structurally sound walls. Walls should be brushed or washed to remove dirt, oil, efflorescence, or other materi-als that may reduce the bond between the coating and the wall.
Sheet membranes and panels (drainage boards) are less dependent on workmanship and on surface preparation than coatings. Many of the membrane systems are better able to remain intact in the event of settlement or other movement of the foundation wall. All seams, terminations, and penetra-tions must be properly sealed. Care must also be exercised during the backfilling process to ensure that the barrier is not damaged.
In crawl space and stem walls, which typically are not treated on the exterior to prevent water entry as basement walls are, crack control measures become more important. In these cases, termites can enter the block through small cracks and move unseen up ungrouted cores. In these instances, solid grouting or capping of the walls is recommended.
Capping Concrete Masonry Walls
Various methods are used to seal the tops of masonry foundation walls. Should termites penetrate the face shell of a concrete masonry wall below, the cap prevents them from direct access to the wood superstructure. In reinforced construction, the masonry bond beam at the top of the wall serves as an effective cap, as shown in Figure 3.
Bond beam units are specifically designed to
accommo-date horizontal reinforcement and grout as shown in Figure 4.
Bond beam units can be either solid bottom or open bottom.
The latter requires a screen grout stop or expanded metal to contain the grout within the unit. A reinforced bond beam is preferred to solid units or solid bottom units with solid head joints since the reinforcement in bond beams will hold any cracks that form tightly together to prevent termite entry through the cracks.
Proper grouting procedures are important to ensure bond with the masonry units and void free areas in bond beams and cells to be filled. Grout should conform to the Specification for Grout for Masonry, ASTM C 476 (ref. 7) or be specified to have a minimum compressive strength of 2,000 psi (13.8 MPa) at 28 days in accordance with the Specification for Masonry Structures, ACI 530.1/ASCE 6/TMS 402 (ref. 6).
The Specification also requires enough water in the grout mixture to achieve a slump of 8 to 11 inches (203 to 279 mm) (ref. 6, 4) when tested in accordance with ASTM C 143 Standard Test Method for Slump of Hydraulic Cement Concrete (ref. 9). See Figure 5.
This high slump is contrary to the principles of cast-in-place concrete where high slump levels lead to reduced strengths and higher shrinkage.
Many engineers mistakenly try to apply this same analogy to masonry – lowering the water content in an effort to reduce shrinkage potential. How-ever, in masonry construction, the high slump is critical as it allows the grout to be fluid enough to flow around reinforcement and completely fill all the voids (ref. 3, 4, and 6). The initial high water-to-cement ratio is reduced significantly as the masonry units absorb the excess water, resulting in higher strengths and low shrinkage properties despite the high initial water-to-cement ratio. Ad-ditionally, as the excess water is absorbed into the masonry units, some of the cement is drawn into the unit with the water creating excellent bond and reducing the formation of voids.
Grout should also be placed in lifts not exceeding 5 ft. (ref. 6). A lift is the layer of grout placed in a single continuous operation. Addition-ally, each lift should be consolidated with either a 3/4 in. (19 mm) diameter low velocity vibrator. Consolidation eliminates voids, helping to ensure complete grout fill and good bond with the masonry units. After the water is absorbed from the grout mixture into the masonry (normally 3 to 10 minutes after placement, depending on the absorption characteristics of the unit and weather conditions), the grout should be re-consolidated to close the space left by the excess water that was absorbed (ref. 3). In any case, reconsolidation must be completed before the grout loses its plasticity.
Metal termite shields may be installed as a continuous barrier directly below the sill plate. If infestation occurs, ter-mites are forced to build conspicuous access tunnels around the shield, making detection easy. Because termites require only a 1/32 inch (0.79 mm) gap for penetration, termite shields must be installed with great care to be effective. All seams must be soldered and all openings around anchor bolts and service lead-ins must be sealed. Because of the extreme care required to provide an impenetrable metal termite shield, they generally are not to be relied on for termite protection.
Exterior Insulation
The rigid plastic foams that are often used to insu-late crawl space and the exterior side of basement walls can allow termites to create undetectable tunnels and is prohibited for such use by some codes (ref. 7). An advantage of concrete masonry foundation walls is their ability to accommodate CLOSEd BOTTOM OPEN BOTTOM
Figure 4—Bond Beam Units for Reinforced Construction Figure 3—Masonry Bond Beam Cap
CLOSEd BOTTOM OPEN BOTTOM
CONTINUOUS REINFORCEMENT AS REQUIREd
GROUT STOP MATERIAL
BONd BEAM COURSE WOOd SILL FLOOR JOIST
FLOOR SHEATHING
8 in. (203 mm) MINIMUM EXPOSEd WOOd SHEATHING
FINISH GRADE
89
Additional Considerations for Crawl Spaces
Figure 6 illustrates termite control measures for crawl space foundations. Crawl space floors should be kept at or above the exterior finished grade to facilitate drainage in the crawl space. Where this is not possible, or on sites where water flows toward the building due to the site slope, area drains should be installed. Unless specified otherwise by local codes, wood girders should be at least 12 inches (305 mm) above the crawl space floor, and wood joists should be no closer than 18 inches (457 mm) to the soil. In all cases, enough clearance should be maintained to allow access to the crawl space for inspection.
ChEMICAl TREATMENTS
Numerous methods are available to create a pesticide barrier within the soil adjacent to a structure to prevent termite entry.
Soil treatment before or during construction is often most effective as there is better access to the subgrade soil. If a slab-on-grade is also going to be used, the soil under the slab can also be pretreated. While post-construction treatment is far more common, it is also more difficult. Limited access to some areas may not allow for an effective chemical barrier to be established.
Figure 6—Termite Control Measures for Crawl Space Foundations Figure 5—Masonry Requires a Fluid Grout;
Slump to be between 8 and 11 in. (ref. 6) TO 279 mm) SLUMP
8 in. (203 mm)
12 in. (305 mm) CONE 4 in. (102 mm)
8 TO 11 in. (203
FLOOR JOIST FLOOR SHEATHING
8 in. (203 mm) MINIMUM EXPOSEd WOOd
SHEATHING
FINISH GRADE
DESIRED GRADE LEVEL
12 in. (305 mm) MININUM 18 in. (457 mm) MINIMUM
24 in. (610 mm) dESIRABLE
REINFORCING STEEL AS REQUIREd
OPTIONAL AREA dRAIN AT LOW POINT
OPTIONAL FOOTING DRAIN WHERE CRAWL SPACE GRADE IS BELOW EXTERIOR GROUND LEVEL
WOOD GIRDER
insulation within the cores of the masonry units where it is protected from direct contact with the soil. Either rigid foam insulation inserts, granular fill insulation, or foamed-in-place insulation can be used for this purpose.
NATIONAL CONCRETE MASONRY ASSOCIATION To order a complete TEK Manual or TEK Index, 2302 Horse Pen Road, Herndon, Virginia 20171-3499 contact NCMA Publications (703) 713-1900 www.ncma.org
CONClUSION
Concrete masonry is an ideal construction material to resist termites. It does not provide food to attract them, and provides a barrier to prevent termite entry. It is also very versatile
REFERENCES
1. Basement Manual, TR-68B. National Concrete Masonry Association, 2000
2. Concrete Masonry Homes: Recommended Practices. U.S. Department of Housing and Urban Development, Office of Policy development and Research, 1999.
3. Grouting Concrete Masonry Walls, NCMA TEK 3-2. National Concrete Masonry Association, 1997.
4. Grout for Concrete Masonry, NCMA TEK 9-4. National Concrete Masonry Association, 1998.
5. Guide to Residential Cast-In-Place Concrete Construction, ACI 332-84. American Concrete Institute, 1984.
6. Specification for Masonry Structures, ACI 530.1-99/ASCE 6-99/TMS 602-99. Reported by the Masonry Standards Joint Committee, 1999.
7. Standard Building Code. Southern Building Code Congress International, 1999: 2304.1.4.
8. Standard Specification for Grout for Masonry, ASTM C 476-99. American Society for Testing and Materials, 1999.
9. Standard Test Method for Slump of Hydraulic Concrete, ASTM C 143/C 143M. American Society for Testing and Mate-rials, 1998
with an almost endless amount of architectural shapes, sizes, textures, and colors available. An innovative, totally termite proof concrete masonry floor system utilizing a hidden steel bar joist supporting system is also available.
Provided by:
91 Disclaimer: NCMA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication.
INTRODUCTION
The metric system (Systeme Internationale or SI system) is the standard international system of measurement, and the system that has been mandated by the Metric Conversion Act (ref. 1) for use in the construction of all United States Federal buildings. Essentially, the Metric Conversion Act requires building designs and construction drawings to be submitted in metric units and constructed according to metric specifi cations.
The subsequent Savings in Construction Act of 1996 (ref. 5) places strong limitations for Federal agencies requiring hard metric concrete masonry units and allows conventional con-crete masonry units to be used in metric construction projects.
Economical adjustments that have virtually no impact on the confi guration of the fi nal project can be made to accommodate inch-pound units on a job where the plans and specifi cations are in metric, as described here and in Metric Design Guidelines for Concrete Masonry Construction (ref. 3).
Complying with these government mandates requires a knowledge of the metric system of measurement and its conventions as well as an understanding of how construction materials, such as concrete masonry, are best incorporated into a metric building design.
Table 1—Metric Decimal Prefi xes
Order of
Prefi x Symbol magnitude Expression milli m 10-3 one thousandth, 0.001 centi c 10-2 one hundredth, 0.01
deci d 10-1 one tenth, 0.1
deca da 10 ten, 10
hecto h 102 one hundred, 100
kilo k 103 one thousand, 1000
mega M 106 one million, 1,000,000
TEK 3-10A © 2008 National Concrete Masonry Association (replaces TEK 3-10)